For the last 25 years, work has progressed on manufacturing drawn cans for food product. These containers were made of materials such as aluminum and low temper steels in order to facilitate the drawing operation. In addition to this the containers usually had a height about equal to or less than the diameter of the container and were fashioned in one or two drawing operations.
Only recently has it been possible to make multiple drawn two piece food containers which were fashioned from organically precoated tin free steel such that postcoating or post treatment operations were not necessary. More particularly, a 24 oz. 404.times.307 tin free steel container was made in a two draw operation. (The can makers convention gives the diameter across the completed doubleseam in inches plus sixteenths of an inch then the height in inches plus sixteenths of an inch. Therefore, the foregoing container is 4 4/16" in diameter by 3 7/16" in height). It is desired to be able to make a container whose height is appreciably greater than the diameter, using precoated starting material in a multiple draw process. It is also desired to make such a container in the popular 16 oz. 303.times.406 size or the 15 oz. 300.times.407 size or the 11 oz. 211.times.400 size.
A triple draw process is required to make the foregoing containers, and that process tends to thicken the area of the container side wall near the open end. The amount of thickening increases from the bottom of the container to the top and all the way to the tip of the flange. This thickening is a consequence of the drawing of the material from a flat disc-shape and the variable circumferential compression of the material as a function of its distance from the bottom of the ultimately formed cup. The additional material thickness at the top of the container serves no useful purpose, and is a waste of material increasing the weight and cost of the container.
The preferred container is fashioned from double reduced plate and more specifically from plate of DR9 temper and about 65# per base box base weight. DR9 is a tin mill product specification which relates to the process by which the metal is cold reduced in two stages with an anneal preformed between the two cold rolling operations. The steel is reduced approximately 89% in the first reduction, is annealled, and then is reduced about 25 to 40% in the second and final cold reduction. The base box terminology for base weight is standard in the can making industry; it originally referred to the amount of steel in a base box of tinplate consisting of 112 sheets of steel 14".times.20", or 31,360 square inches plate. Today the base box as related to base weight refers to the amount of steel in 31,360 square inches of steel, whether in the form of coil or cut sheets. The preferred embodiment can be made from tin free steel (TFS), tinplate, nickel plated steel, or steel base material.
This material may be coated on what ultimately will be the outside surface by an epoxide-resin-type or an organosol coating. The inside may be coated with a coating consisting of a combination of resins of the organosol type. Inside and outside coatings are capable of withstanding the drawing and ironing stresses typical of can-making operations. Consequently, the container can be made from a relatively high temper material and should not require a postcoating. Of course, tinplate which is not organically cated will require at least an internal postcoating operation.
The outside coating is applied by roller coating or coil coating and cured in an oven. For sheet coating operations, this coating is baked in a temperature range of 300.degree. to 400.degree. for about 6 to 10 minutes. It is usually applied to the metal substrate at a film weight of 8 to 15 mg per 4 square inches of plate area. The outside coating can be of several chemical types such as a vinyl organosol, an epoxide resin, an amine resin, a phenolic resin or suitably formulated blends of these resins. The inside coating is generally applied at a film weight of 15 to 35 mg per 4 square inches of plate area; that coating can be either sheet coated or coil coated. A baking temperature of 300.degree. to 400.degree. F. for 8 to 10 minutes is generally used in sheet coating. Inside coatings contain mixtures of phenolic resin, epoxy resin, vinyl solution resins of the vinyl acetate-vinyl chloride copolymer type and high molecular weight polyvinyl chloride dispersion resins.
The preferred method used in order to produce such a desired container having a minimum amount of the high temper DR9 steel, includes three drawing operations which may take place in a press such as that disclosed in U.S. Pat. No. 4,262,510 which is assigned to the same Company as the present invention. For a triple drawn and ironed can the diameter of the container and the wall thickness are concurrently reduced in each forming operation. More specifically, the first operation blanks and forms the sheet of precoated material into a shallow cup wherein the diameter is in excess of the height. During this operation the wall thickness is reduced by ironing while drawing such that part of the wall is reduced to less than the thickness of an unironed container. The second operation redraws the container and reduces the diameter and again concurrently irons the wall to similarly reduce thickness from the top to the bottom. In this second operation the diameter is reduced and the height increased so that they are about equal. The final operation reduces the diameter still further and once again concurrently irons the side wall to produce a preferred thinness and uniformity such that the container achieves its final configuration with a sidewall which is about 0.001" less than the starting gauge before bottom profile and sidewall beading.
In any of the multiple operations where the diameter is reduced and the side wall is thinned the ironing operation may be stopped before it reaches the flange. Consequently, the flange thickness as well as the side wall area next adjacent the flange can be left thicker. It should be appreciated that a complete container can be manufactured from precoated stock without having the need for any washing, repair postcoating or additional energy-intensive operations.
The addition of ironing to the multiple-draw process permits the original cutedge or circular blank to have a smaller diameter than that necessary for an unironed similar size container. Therefore, the amount of steel used for this container is less than that needed for drawn containers of the same size. This reduction in steel saves material and reduces the ultimate container weight.
During forming at high levels of pressure, heat is generated. Lubrication topically applied to the coating is a critical aspect for forming multiple drawn and ironed containers. The lubricant provides the needed slip properties when precoated plate is formed in the press tooling. Without proper lubrication, the coatings will be scraped off by the press tools resulting in scuffing, drawing failures and possible damage to the punches and dies in the press. Lubricants such as Boler wax, lanolin or petrolatum can be used. For multiple drawn containers, petrolatum is the best with regard to tool lubrication, good flavor performance, price and stability. The lubricant can be topically applied by spraying from standard spray guns, fogging by special electrostatic machines over the coated plate or by mixing the lubricant into the coating.
The lubricant must be able to work under both the heat and pressure in order to protect the coating and metal combination from destruction. The mechanical working of the precoated metal in the dies of the press causes a rise in temperature of the precoating and metal as they are formed into containers. Temperatures in the press tooling and consequently in the containers at least at the interface rise to 150.degree. F. in the first redraw station and reach as high as 200.degree. F. in the second redraw station, but temperatures as high as 280.degree. F. have been measured. In addition to or instead of topical lubricants dry film type lubricants can be dispersed in solvent and incorporated in the coating. During the forming operations, the dry film lubricant becomes available at the heated interface as a hard, solid protective layer. It is essential that the melting point of the solid lubricant be adjusted to cooperate with the levels of heat existing during the multiple forming steps whereby the lubricant first becomes available in a flowable form at the time when the temperature exceeds a predetermined level.
A working temperature is ultimately arrived at during the multiple forming operations and contrary to drawing and ironing of beverage containers there is no coolant/lubricant flood of the containers and tooling used to form sanitary food cans. The flooding of the containers and tooling requires that the containers be cleaned by washing and drying after forming. Here, there is disclosed an essentially clean dry process which provides a container which is ready to be packed and processed. Of course, the foregoing relates primarily to organically precoated stock and not necessarily to unorganically coated tinplate. The working temperature is a result of the process parameters, the tooling design, material used and other factors that influence the pressure applied during forming.
Traditionally, any variation in plate gauge, hardness or temper which affected the drawability had to be overcome by different punch and die dimensions. In particular, a few 0.0001" in the clearance between the punch and die (for a drawn and ironed container diameter in the range of 21/2 to 5") could substantially affect the outcome of a drawing and ironing process. Metal tends to be a resistant to thinning during the plastic deformation resulting from drawing. In a multiple draw/redraw process with ironing, the resistance to thinning will affect the ultimate container volume because as the metal is thinned it elongates resulting in greater side wall height. Similarly, the plate gauge varies throughout a coil thus affecting the ultimately container size. In a two-piece container the height and volume are critical in that each container must be of uniform size in order to properly pass through existing conveyors, processing and labeling equipment.
From the foregoing it is clear that the process used to multiple form drawn and ironed food containers generates a sufficient amount of heat and working pressure to cause uncontrolled dimensional changes in the tooling. These changes are critical to the overall container shape and more specifically, to the variations in ultimate height, volume, side wall condition, bottom profile integrity and flange length before trimming from one container to another. The untrimmed flange length at any given circumferential portion thereof is also a function of the original material gauge and the grain direction established during the rolling of the sheet. Consequently, if the metal is high earing the flange will be extended radially at all points which are about 45.degree. to the grain direction to an extent which is wasteful of material and harmful to the process. Conversely, low earing metal will not extend as far. Light gauge metal will tend to have a short or narrow flange in a radial direction normal to the direction of the grain. This minimum radial extent could result in incomplete trimmed rings such that they will be unmanageable and/or the flange too short.
Also, low temper steel and/or a heavy plate gauge and/or plate with low levels of lubricants produce large flanges causing wrinkling about the circumferential flange periphery. That wrinkling has difficulty in flowing past the clamping sleeve through the tooling between the punch and die. More particularly, the uncontrolled wrinkling of the flange periphery locks against the clamping sleeve which is designed to control the feed of the metal to a prescribed rate. Locking puts excessive stress on the side wall during drawing and/or on the bottom during profiling. That stress causes tearouts in the side wall and breakouts in the bottom wall. More specifically, the feeding of the material into the die as a result of being drawn by the punch is not uniform and not controlled because of the locking due to wrinkling about the extended flange periphery.
In a high speed draw/redraw food container multiple forming operation at speeds of 100 containers per minute or higher the variables which will determine the quality of the container produced are many and are changing with respect to time. It is therefore essential to such a commercial operation to be able to accurately control the process and consequently the results by some means. It is the object of the disclosure to present the technique, method and apparatus discovered which permits the stated problems to be resolved.